RECLAMATION OF TWO SALT-AFFECTED SOILS
56 Proceedings of The South African Sugar Technologists' Association — June 1969 not provide any explanation for the improved growth
on the sulphur plot. The treatments may, however, have influenced plant growth in other ways, for example by affecting nitrogen mineralisation. Sindhu and Cornfield4 found in a laboratory study that mincralisable N was particularly reduced on reclam- ation of an alkali soil with gypsum, and suggested that this was due to a considerable loss of easily mincralisable organic nitrogen compounds.
As found in the leaching tests discussed later in this paper, sulphur may effect a more rapid improve- ment in soil physical conditions than gypsum. The better plant growth may thus have resulted from the enhanced soil aeration and drainage. Another pos- sibility is the fungicidal effecl of the sulphur which may have suppressed soil pathogens in the rhizo- sphere3. To establish precise effects and mechanisms, a full scale trial is clearly necessary.
2. The saline-alkali site
The area involved was 2-3 acres of gently sloping boulder-bed derived sandy clay loam which had been salinized by ground waters percolating down- slope from a storage dam. The degree of saliniza- lion varied considerably over the area, and the data in Table 1 refer to a profile in a more highly salinized section. The entire site has been drained and allowed to lie fallow, but regular sampling over the past 18 months has shown little change in the soil conditions. As it was not possible to undertake a full-scale reclamation programme, a laboratory test is being carried out to examine the effects of various amendments on this soil.
Soil from the depth intervals shown in Table I was air dried and crushed to pass a 2mm sieve. Brass cylinders 2 ft. long by 2 1/2 ins. diameter were packed with this soil in the same order and to the same
depths as in the field. The lower ends of the cylinders were covered with nylon gauze and the cylinders clamped vertically, resting in Buchner funnels. Sul- phur was applied to the surface of one soil at a rate equivalent to 1 ton per acre and gypsum to another at 5 tons per acre and lightly "dug" into the top few inches of soil. A third cylinder received no amendments and acted as a control. Distilled water (200 ml) was added to each column, additions of 50 ml being made whenever the free water had drained away. The volume of leachate was measured weekly and analysed as soon as a sufficient quantity had been collected.
Results
The rate at which water was added to the columns is illustrated in Figure 2, together with the outflow data. Table II gives analyses of the leachate samples.
Discussion and conclusions
Although initially more rapid on the gypsum treated soil (soil 2), the rate of acceptance of water by the sulphur treated column (soil 1) increased gradually to exceed that of soil 2 after five months.
Outflow from column 1 commenced several weeks earlier than from column 2 and continued at a far greater rate. Generally the sulphur appears to have effected a more rapid improvement in the soil physi- cal conditions. It is noteworthy that, during the first nine months of the test, no leachate has been collected from the control column.
The quantity of salts removed from soil 1 was considerable at first, the total sodium leached from this column being equivalent to 7.7 g NaCl (total sodium, present in the column initially was equiva- lent to 22.7 g NaCl). The rate of removal of salts TABLE I
Analysis of the saline-alkali soil profile at various depths
TABLE II
Analysis of leachate from the saline-alkali soil columns treated with sulphur or gypsum
decreased steadily, however, while the pH of the leachate increased from 4.1 to 9.4 over the three months.
Leachate from the gypsum column was also rich in salts, but this treatment was clearly less effective.
Based on results to date, it appears that sulphur should be used to reduce the soil pH and rapidly improve the soil physical condition. Despite its high calcium content, such a soil may develop alkalinity on leaching, and it is suggested that the application of sulphur (1-2 tons per acre) followed by gypsum (4-5 tons per acre) would be the most effective approach in this case. Leaching of a soil so treated should result in the rapid removal of excess sodium and its replacement with calcium, thus bringing about the return of favourable chemical and physical soil conditions.
It is proposed that studies with these leaching columns continue in an effort to forecast responses to various field treatments on similar soils. However, a replicated trial to test the efficacy of the treat- ment proposed above, and others, would clearly be of greatest value.
References
1. Maud, R. R. (1959). The occurrence of two alkali soils in Zululand. Proc. S. Afr. Sug. Technol. Ass. 33: 138- 144.
2. Richards, L. A. ed. (1954). Diagnosis and improvement of saline and alkali soils. U.S.D.A. Agric. Handbook 60.
3. Roth, G. (1969). Personal communication.
4. Sindhu, M. A., and Cornfield, A. H. (1967). Effect of simulated reclamation treatments on chemical proper- ties and nitrogen mineralising characteristics of saline and alkali soils from West Pakistan. J. Sci. Fd. Agric.
18: 507-509.
5. Von der Meden, E. A. (1966). Note on salinity limits for sugarcane in Natal. Proc. S. Afr. Sug. Techno/.
Ass. 40: 273-275.
6. Von der Meden, E. (1967). Problems of saline soils and their management. S. Afr. Sug. J. 5 1 : 750-751.
Discussion
Dr. Thompson: I am surprised at the rapid plant response to elemental sulphur applied to the soil.
Mr. von der Meden: It is quickly oxidised and immediately thereafter reduces the pH in the soil.
Figure I shows that the gypsum had an even quicker effect in reducing pH, however.
Dr. Sumner: Could not the beneficial effect of sulphur, compared to gypsum, on the alkali soil be due to it creating localised areas of very acid soil?
Only an average pH is being measured. The localised areas could be making some trace element more available and thus promoting better plant growth than the gypsum.
Mr. von der Meden: I think such a localised effect on trace elements a most likely possibility.
There is also a possible effect on nitrogen.
Mr. Meyer: The improvement could be due simply to increased sulphur uptake by the plant.
Mr. von der Meden: It is a possibility b u t the land had in the past been fertilized with super- phosphate.
Mr. Rogers: Was any attempt made to drain the soil?
Mr. von der Meden: In the last two years drains have been dug and although they a r e working efficiently they do not appear to have resulted in a change in the soil. At present their main purpose is to prevent more water coming into the saline area.
Mr. Tucker: The situation referred to in this paper came about because of water logging and therefore before anything else drainage should be carried out.
Mr. Odendaal: Was not a section t h a t h a d not been treated at all reaped in order to provide a comparison?
Mr. von der Meden: This was done but n o t in the alkali site which was only twenty yards square.
The surrounding area was saline-alkali and this gave about the same yield as the sulphur treated area.
Dr. Thompson: Another site has been located where it should be possible to put down a statisti- cally designed experiment.
Dr. Dodds: Was the sulphur that was used tested for small quantities of two similar elements — selenium and tellurium?
Mr. von der Meden: No, but it would be in- teresting to do so.
58 Proceedings of The South African Sugar Technologists' Association — June 1969
60 Proceedings of The South African Sugar Technologists' Association - June 1969